International Publication No. 2018/025440 (“PTL 1”) discloses a spark plug including an insulator and a metal shell. PTL 1 discloses that, in the spark plug, the metal shell includes an outer peripheral surface extending from a rear end of a screw portion to a front end of the screw portion, the metal shell includes a portion exposed to combustion gas of an internal combustion engine, and the insulator includes a portion exposed to the combustion gas, and that heat resistance can be improved when the surface area of the portion of the outer peripheral surface of the metal shell, the surface area of the portion of the metal shell, and the surface area of the portion of the insulator satisfy a predetermined relationship.
In recent years, internal combustion engines are increasingly required to have high efficiency. It is known that improving combustion speed is effective to improve efficiency of internal combustion engines. A spark plug with a pre-chamber (hereinafter also referred to as the pre-chamber plug) has been attracting attention in recent years as being effective to improve combustion speed. The pre-chamber plug has been applied to power generators and engines for races, and an improvement of combustion efficiency has been confirmed. Further, even when applied to internal combustion engines other than power generators and engines for races, the pre-chamber plug is effective to improve combustion efficiency.
In pre-chamber plugs, after ignition of a spark between electrodes, combustion first occurs in a pre-chamber. Then, the combustion gas inside the pre-chamber is injected out via through holes (injection holes) that are in communication with the outside, and the injected high-temperature gas as an ignition source causes explosive combustion in a main combustion chamber. The speed of injection of the high-temperature gas from the pre-chamber is higher than that of combustion caused by ignition of a spark plug without a pre-chamber. Moreover, the entire trail of the injected high-temperature gas serves as an ignition source. Therefore, it is possible to cause a large amount of fuel to be brought into contact with the high-temperature gas. Thus, the combustion speed of the pre-chamber plug is higher than the combustion speed of the spark plug without a pre-chamber, and therefore, the effect of improving combustion efficiency can be expected.
However, since pre-chamber plugs have a structure in which a firing end is hermetically closed, the temperature of the inside of the pre-chamber tends to become high. If an overheated state continues, pre-ignition may occur, which is a problem. In contrast, when the temperature of the inside of the pre-chamber becomes too low, combustion inside the pre-chamber consumes energy, so that combustion cannot occur in the main combustion chamber and misfires may occur, which is a problem.